The Complete Gardener's Guide ,
Improve Soil Structure by Adding Organic Material
Improve your soil's structure by working in plenty of bulky, well-rotted organic matter, such as manure.

Knowing how to make the best of your garden’s conditions by nurturing and improving the soil will help you work wonders on your plot—even with challenging soils, like heavy clay or chalk.

Working With Your Soil

Analyzing the basic structure and drainage of your soil, its pH, organic content, and fertility is an important first step. If you dig down into soil, you will eventually see a change from the darker, fertile, topsoil layer to the paler, infertile subsoil layer and rock beneath. Some soils have very little depth, adversely affecting cultivation. On the other hand loams cultivated over many years with regular additions of manure can be very deep. The color difference between topsoil and subsoil is due to the lack of humus or organic matter at depths where earthworms are not active. In clay, the subsoil is orange, but in waterlogged conditions it can be blue-gray due to the lack of oxygen. Never mix subsoil into topsoil; the lower layer can be toxic to plants.

In poor soils the lack of depth can severely affect the growth of both ornamentals and crops. Raised beds allow more topsoil to be added, contained within a frame.

What Is Soil?

Soil is a mixture of ground-up rock particles, organic matter or humus, and countless microscopic organisms and tiny invertebrates. In healthy, well-drained soil the particles are clumped together with the spaces in between occupied by water and air. This crumb structure allows earthworms to move through, creating a network of channels that plant roots can make use of. Earthworms are very active “gardeners,” pulling leaf litter and other organic debris down into the soil where bacteria and fungi, together with other soil organisms, break it down, releasing and recycling nutrients. Creating conditions that benefit these soil organisms is the best way to maintain fertility.

Types of Soil

  • Sandy soils have coarse grains and lack organic matter, which makes them very free-draining. They are easy to dig, can be cultivated all year round, and warm up quickly in spring for early sowings. Self-seeding plants can become a nuisance; they germinate freely, but weeds and unwanted plants can easily be pulled out.
  • Loam soils are ideal, having a good balance of clay particles to retain moisture and nutrients, and sand grains to improve drainage. Loam usually has a healthy worm population, and most plants thrive with little extra feeding. A clay loam will be slower to warm up in spring than a sandy one.
  • Clay soils are some of the most fertile, because clay particles hold on to nutrients. When well drained, they can be dug to break up solid areas and incorporate bulky organic matter like manures and composts—this is the best way to improve poor drainage, a common problem with clay soils.
  • Chalk soils are usually pale and peppered with fragments of flint or limestone, making digging difficult. They tend to be free-draining and low in humus or organic matter. They warm up quickly in spring but can be shallow, overlying solid rock. Chalk also has a high pH, excluding the growth of lime-hating plants. 

Problem Soils

To improve the texture and drainage of heavy clay soils, work in large amounts of bulky organic matter. This chemically alters the clay and causes the fine particles to cluster, opening up drainage channels. Incorporate gravel to improve drainage, and raise the soil level in your beds and borders. Avoid digging or walking on clay when it is wet, and don’t dig when it is dry. Both activities destroy the structure.

Sandy soils are free draining, but this can leach nutrients out of the upper layers and away from the roots of plants, leaving the soil impoverished. They can be improved by the addition of well-rotted organic matter to hold nutrients and moisture in the soil; moisture-loving plants can also be mulched.

To Dig or Not to Dig?

Soil can be damaged in several ways. Compaction destroys air and rooting channels, impairs drainage, and reduces earthworm activity. Digging is then essential to help loosen up compacted and heavy soil. You will also need to turn over uncultivated soil to prepare it for planting. Some soils are easy to dig and can be cultivated all year round, while others, such as heavy, clay soils, may have a relatively narrow window for sowing and planting.

Generally, there is little need for routine digging in a garden that is already cultivated. Worms will do much of the hard work for you, incorporating organic matter added to the surface. Digging shallow soils can be problematic because there is a risk of bringing infertile subsoil to the surface; digging can also disrupt beneficial soil micro-organisms. Digging dry clay can destroy an open, crumblike structure and turn it to dust that sets like concrete after rain. Walking on or digging wet clay also damages its structure.

Types of Composts

  • Soil-based composts, such as John Innes, are the best choice for the long-term health of perennials, climbers, shrubs, and trees grown in containers. Compared with peat or peat-substitute based mixtures, they retain their structure better and don’t shrink or compact over time.
  • Multipurpose composts are ideal for short-term planting such as summer patio pots, and retain moisture and nutrients well. They weigh far less than soil-based media, allowing pots to be mored more easily.
  • Ericaceous or lime-free composts, based on loam or peat, are used for shrubs, such as camellias and rhododendrons, that require acid soil. Loam soils are ideal, having a good balance of clay particles to retain moisture and nutrients, and sand grains to improve drainage. Loam usually has a healthy worm population, and most plants thrive with little extra feeding. A clay loam will be slower to warm up in spring than a sandy one.
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